[petsc-users] KSP convergence problem

[John Mousel]

Mark,

This is a flow solver Poisson equation. Is the coordinate functionality
applicable?

John


On Wed, Mar 20, 2013 at 6:04 PM, Mark F. Adams <mark.adams at columbia.edu>wrote:

>
> On Mar 20, 2013, at 6:14 PM, John Mousel <john.mousel at gmail.com> wrote:
>
> Can you comment on a GASM type approach to find a solution for the null
> space? I notice that the null vectors that successfully make the true
> residual drop are only complicated in a very thin band around the
> interface. This band is easy to identify using a level set. Other than
> that, the null space vector has a low frequency variation. My thought was
> to break the matrix into two sub-matrices, and somehow apply GAMG as a
> preconditioner on the far matrix, and ILU on the interface-adjacent matrix.
> Is this dumb or a complete misunderstanding of GASM?
>
>
> The null space in GAMG is not a projection, it does not have to be exact.
>  Do you try using the 6 RBM or giving GAMG coordinates?
>
> Also, you might try not smoothing the (-pc_gamg_nsmooths 0).  Unsymetric
> matrices can work better this way.
>
>
>
> On Wed, Mar 20, 2013 at 5:08 PM, Jed Brown <jedbrown at mcs.anl.gov> wrote:
>
>>
>> On Wed, Mar 20, 2013 at 5:04 PM, John Mousel <john.mousel at gmail.com>wrote:
>>
>>> I've wanted to scrap this approach for a long time, but moving away from
>>> these GFM-type treatments is not a choice that I've been allowed to follow
>>> through on for various reasons which are out of my control.
>>
>>
>> Unless there are some clever tricks to characterize the null space or to
>> keep preconditioners compatible with the null space, the folks making the
>> decisions might have to reconsider. It doesn't matter how sexy a method
>> looks if it requires a solve and that solve cannot be done efficiently.
>>
>
>
>
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